Kinetic studies with tRNAPhe which has been highly purified from normal and malignant cells have shown that there are slight differences in rates of aminoacylation of isoacceptors with and without Wye base and that the rate of transfer of phenylalanine into protein is about the same for these isoacceptors. Patterns of codon recognition in a number of species have shown that evolutionary changes have occurred between plants and animals, and between lower and higher animals, and provide further examples of changes between procaryotes and eucaryotes. Aminoacylation of 19 tRNAs from seven species under homologous and heterologous conditions have shown that the greatest number of evolutionary changes have occurred between procaryotes and eucaryotes and the fewest number between higher eucaryotes.